Hi there,
Thanks for posting on our forum.
I doubt that the version of MSFT server would be the direct cause; but it's more likely a specific configuration within the guest itself causing an offline backup to occur (which you should be able to alter).
The following is a set of known conditions required to make sure that an online backup is possible that you can check against your system:
https://www.backupassist.com/education/ ... ckups.html.
Hopefully this information helps.